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Dry and itchy foreskin often indicates yeast infection. If your partner is having exces white discharge or even without discharge may likely to have the yeast infection and that may cause itching of your foreskin.
Any other irritant substances like soaps or other chemicals also may sometimes cause irritation.
If it is due to Yeast infection then you and your girlfriend together to take treatment. 1-2% Clotrimazole
cream for external application and Tablet Flucanazole 150 mg weekly once for 3-4 weeks for you and clotrimazole vaginal pessaries 200 mg daily for 6 days for your girl friend andTab. Flucanazole weekly once for 3-4 weeks will clear all your symptoms.
Before that you visit your physician and confirm the diagnosis and with his guidance you have your treatment.